Taste transduction is one of the most sophisticated forms of chemotransduction in animals (see, e.g., Margolskee, BioEssays 15:645–650 (1993); Avenet & Lindemann, J. Membrane Biol. 112:1–8 (1989)). Gustatory signaling is found throughout the animal kingdom, from simple metazoans to the most complex of vertebrates; its main purpose is to provide a reliable signaling response to non-volatile ligands. Each of these modalities is though to be mediated by distinct signaling pathways mediated by receptors or channels, leading to receptor cell depolarization, generation of a receptor or action potential, and release of neurotransmitter at gustatory afferent neuron synapses (see, e.g., Roper, Ann. Rev. Neurosci. 12:329–353 (1989)).
Mammals are believed to have five basic taste modalities: sweet, bitter, sour, salty and unami (the taste of monosodium glutamate) (see, e.g., Kawamura & Kare, Introduction to Unami: A Basic Taste (1987); Kinnamon & Cummings, Ann. Rev. Physiol. 54:715–731(1992); Lindemann, Physiol. Rev. 76:718–766 (1996); Stewart et al., Am. J. Physiol. 272:1–26 (1997)). Extensive psychophysical studies in humans have reported that different regions of the tongue display different gustatory preferences (see, e.g., Hoffmann, Menchen. Arch. Path. Anat. Physiol. 62:516–530 (1875); Bradley et al., Anatomical Record 212: 246–249 (1985); Miller & Reedy, Physiol. Behav. 47:1213–1219 (1990)). Also, numerous physiological studies in animals have shown that taste receptor cells may selectively respond to different tastants (see, e.g., Akabas et al., Science 242:1047–1050 (1988); Gilbertson et al., J. Gen. Physiol. 100:803–24 (1992); Bernhardt et al., J. Physiol. 490:325–336 (1996); Cummings et al., J. Neurophysiol. 75:1256–1263 (1996)).
In mammals, taste receptor cells are assembled into taste buds that are distributed into different papillae in the tongue epithelium. Circumvallate papillae, found at the very back of the tongue, contain hundreds (mice) to thousands (human) of taste buds and are particularly sensitive to bitter substances. Foliate papillae, localized to the posterior lateral edge of the tongue, contain dozens to hundreds of taste buds and are particularly sensitive to sour and bitter substances. Fungiform papillae containing a single or a few taste buds are at the front of the tongue and are thought to mediate much of the sweet taste modality.
Each taste bud, depending on the species, contain 50–150 cells, including precursor cells, support cells, and taste receptor cells (see, e.g., Lindemann, Physiol. Rev. 76:718–766 (1996)). Receptor cells are innervated at their base by afferent nerve endings that transmit information to the taste centers of the cortex through synapses in the brain stem and thalamus. Elucidating the mechanisms of taste cell signaling and information processing is critical for understanding the function, regulation, and “perception” of the sense of taste.
Although much is known about the psychophysics and physiology of taste cell function, very little is known about the molecules and pathways that mediate these sensory signaling responses (reviewed by Gilbertson, Current Opn. in Neurobiol. 3:532–539 (1993)). Electrophysiological studies suggest that sour and salty tastants modulate taste cell function by direct entry of H+ and Na+ ions through specialized membrane channels on the apical surface of the cell. In the case of sour compounds, taste cell depolarization is hypothesized to result from H+ blockage of K+ channels (see, e.g., Kinnamon et al., Proc. Nat'l Acad. Sci. USA 85: 7023–7027 (1988)) or activation of pH-sensitive channels (see, e.g., Gilbertson et al., J. Gen. Physiol. 100:803–24 (1992)); salt transduction may be partly mediated by the entry of Na+ via amiloride-sensitive Na+ channels (see, e.g., Heck et al., Science 223:403–405 (1984); Brand et al., Brain Res. 207–214 (1985); Avenet et al., Nature 331: 351–354 (1988)).
Sweet, bitter, and unami transduction are believed to be mediated by G-protein-coupled receptor (GPCR) signaling pathways (see, e.g., Striem et al., Biochem. J. 260:121–126 (1989); Chaudhari et al., J. Neuros. 16:3817–3826 (1996); Wong et al., Nature 381: 796–800 (1996)). Confusingly, there are almost as many models of signaling pathways for sweet and bitter transduction as there are effector enzymes for GPCR cascades (e.g., G protein subunits, cGMP phosphodiesterase, phospholipase C, adenylate cyclase; see, e.g., Kinnamon & Margolskee, Curr. Opin. Neurobiol. 6:506–513 (1996)). However, little is known about the specific membrane receptors involved in taste transduction, or many of the individual intracellular signaling molecules activated by the individual taste transduction pathways. Identification of such molecules is important given the numerous pharmacological and food industry applications for bitter antagonists, sweet agonists, and modulators of salty and sour taste.
The identification and isolation of taste receptors (including taste ion channels), and taste signaling molecules, such as G-protein subunits and enzymes involved in signal transduction, would allow for the pharmacological and genetic modulation of taste transduction pathways. For example, availability of receptor and channel molecules would permit the screening for high affinity agonists, antagonists, inverse agonists, and modulators of taste cell activity. Such taste modulating compounds could then be used in the pharmaceutical and food industries to customize taste. In addition, such taste cell specific molecules can serve as invaluable tools in the generation of taste topographic maps that elucidate the relationship between the taste cells of the tongue and taste sensory neurons leading to taste centers in the brain.